1. Field of the Invention
The present invention relates to transmission structure of a power unit for a vehicle where a crankshaft, a main shaft and a countershaft provided with plural gear sets, which can be selectively established between the countershaft and the main shaft, where these shafts are all rotatably supported by a crankcase such that axes thereof are mutually parallel. A clutch mechanism is provided between the crankshaft and the main shaft, and is installed at an end of the main shaft outside the crankcase. A shift drum for driving shift forks for selectively establishing a selected gear is provided, and both ends of the shift drum are supported by the crankcase so that the shift drum can be rotated about an axis parallel to the crankshaft, the main shaft and the countershaft.
2. Description of the Background Art
It is known to provide a power unit for a vehicle in which a clutch mechanism is installed at one end of a main shaft of a transmission. In the known power unit, the transmission is provided with multiple gear sets and a shift drum for driving shift forks, and is assembled in a crankcase from a side opposed to the clutch mechanism. Such a power unit is disclosed, for example, in JP-A No. 77858/1998.
However, since in the power unit disclosed in JP-A No. 77858/1998, in a power unit in which a shift drum is assembled in a crankcase from the side opposed to a clutch mechanism, a sealed structure dedicated to the shift drum is required. As a result, the number of parts is increased, and the time required for assembly is increased.
The invention is made in view of such a situation, and the object is to provide the transmission structure of a power unit for a vehicle in which no sealed structure dedicated to a shift drum is required, whereby the number of parts can be reduced and the manhours of assembly can be reduced.